While this is done for all bitmaps, the original use case in mind was for CPU masks and cpulist_parse(). Credit to Yury who suggested to push it down from CPU subsys to bitmap - it simplified things a lot.
It seems that a common configuration is to use the 1st couple cores for housekeeping tasks, and or driving a busy peripheral that generates a lot of interrupts, or something similar. This tends to leave the remaining ones to form a pool of similarly configured cores to take on the real workload of interest to the user. So on machine A - with 32 cores, it could be 0-3 for "system" and then 4-31 being used in boot args like nohz_full=, or rcu_nocbs= as part of setting up the worker pool of CPUs. But then newer machine B is added, and it has 48 cores, and so while the 0-3 part remains unchanged, the pool setup cpu list becomes 4-47. Deployment would be easier if we could just simply replace 31 and 47 with "N" and let the system substitute in the actual number at boot; a number that it knows better than we do. No need to have custom boot args per node, no need to do a trial boot in order to snoop /proc/cpuinfo and/or /sys/devices/system/cpu - no more fencepost errors of using 32 and 48 instead of 31 and 47. Cc: Yury Norov <yury.no...@gmail.com> Cc: Peter Zijlstra <pet...@infradead.org> Cc: "Paul E. McKenney" <paul...@kernel.org> Signed-off-by: Paul Gortmaker <paul.gortma...@windriver.com> --- .../admin-guide/kernel-parameters.rst | 4 ++++ lib/bitmap.c | 18 +++++++++++++----- 2 files changed, 17 insertions(+), 5 deletions(-) diff --git a/Documentation/admin-guide/kernel-parameters.rst b/Documentation/admin-guide/kernel-parameters.rst index 5e080080b058..668f0b69fb4f 100644 --- a/Documentation/admin-guide/kernel-parameters.rst +++ b/Documentation/admin-guide/kernel-parameters.rst @@ -68,6 +68,10 @@ For example one can add to the command line following parameter: where the final item represents CPUs 100,101,125,126,150,151,... +The value "N" can be used as the end of a range, to represent the numerically +last CPU on the system, i.e "foo_cpus=16-N" would be equivalent to "16-31" on +a 32 core system. + The following convenience aliases are also accepted and used: foo_cpus=all diff --git a/lib/bitmap.c b/lib/bitmap.c index a1010646fbe5..d498ea9d526b 100644 --- a/lib/bitmap.c +++ b/lib/bitmap.c @@ -571,7 +571,7 @@ static const char *bitmap_find_region_reverse(const char *start, const char *end return end; } -static const char *bitmap_parse_region(const char *str, struct region *r) +static const char *bitmap_parse_region(const char *str, struct region *r, int nmaskbits) { str = bitmap_getnum(str, &r->start); if (IS_ERR(str)) @@ -583,9 +583,15 @@ static const char *bitmap_parse_region(const char *str, struct region *r) if (*str != '-') return ERR_PTR(-EINVAL); - str = bitmap_getnum(str + 1, &r->end); - if (IS_ERR(str)) - return str; + str++; + if (*str == 'N') { + r->end = nmaskbits - 1; + str++; + } else { + str = bitmap_getnum(str, &r->end); + if (IS_ERR(str)) + return str; + } if (end_of_region(*str)) goto no_pattern; @@ -628,6 +634,8 @@ static const char *bitmap_parse_region(const char *str, struct region *r) * Syntax: range:used_size/group_size * Example: 0-1023:2/256 ==> 0,1,256,257,512,513,768,769 * Optionally the self-descriptive "all" or "none" can be used. + * The value 'N' can be used as the end of a range to indicate the maximum + * allowed value; i.e (nmaskbits - 1). * * Returns: 0 on success, -errno on invalid input strings. Error values: * @@ -656,7 +664,7 @@ int bitmap_parselist(const char *buf, unsigned long *maskp, int nmaskbits) if (buf == NULL) return 0; - buf = bitmap_parse_region(buf, &r); + buf = bitmap_parse_region(buf, &r, nmaskbits); if (IS_ERR(buf)) return PTR_ERR(buf); -- 2.17.1